Module 5
Plate Tectonics & Regional Features
A. Inside of The Earth
Understanding earth’s surface requires knowing what is inside the Earth. You can
directly observe the surface of Earth, but what is down below, in the subsurface? Earth
consists of concentric layers that have different compositions. The outermost layer is the
crust, which includes continental crust and oceanic crust. Beneath the crust is the mantle,
Earth’s most voluminous layer. The molten outer core and the solid inner core are at
Earth’s center.
Continental crust has an average composition similar to this granite. Continental
crust, the thin, light-gray layer on the figure to the right, averages 35 to 40 km (20 to 25
mi) in thickness. Oceanic crust exists beneath the deep oceans and has an average
composition that is the same as basalt, a common dark lava rock. Oceanic crust has an
average thickness of about 7 km (4 mi), which is much thinner than can be shown here
(the barely visible dark-gray layer). The mantle extends from the base of the crust down
to a depth of 2,900 km (1,800 mi). Much of the upper mantle is composed of the green
mineral olivine, as exposed in the center of this rock brought to the surface in a volcano.
The lower mantle has a composition similar to the upper mantle, but it contains minerals
formed at very high pressures. Nearly all of the mantle is solid, not molten. High
temperatures cause some parts to be partially molten, while other parts flow because they
are weak solids. Based on studies of earthquakes, observations of meteorites, and models
for the density of Earth, geoscientists interpret the core to consist of metallic iron and
nickel, such as that observed in iron-nickel meteorites. The outer core is molten, but the
inner core is solid.
In addition to layers with different compositions, Earth has layers that are defined
by strength and by how easily the material in the layers fractures or flows when subjected
to forces. The uppermost mantle is relatively strong and solidly attached to the over lying
crust. The crust and uppermost mantle together form a rigid upper rigid layer called the
lithosphere (lithos means “stone” in Greek), which averages about 100 km (about 60 mi)
in thickness. The part of the uppermost mantle that is in the lithosphere is the lithospheric
mantle. The mantle directly beneath the lithosphere is mostly solid, but it is hotter than
the rock above and can flow under pressure. This part of the upper mantle, called the
asthenosphere, functions as a soft, weak zone under which the lithosphere moves. The
word asthenosphere is from a Greek term for “not strong.” In most regions, the
asthenosphere is approximately 80 km to more than 150 km thick, so it can be deeper
than 250 km.
Why is the Gulf Coast of Texas near sea level, while the Colorado mountains are
3 to 5 km (2 to 3 mi) above sea level? Why are the continents mostly above sea level, but
the ocean floor is below sea level? The primary factor controlling the elevation of a
region is the thickness of the underlying crust. The crust is less dense than the underlying
mantle, and so it rests, or floats, on top of the mantle. The underlying litho spheric mantle
is mostly solid, not liquid. The thickness of continental crust ranges from less than 25 km
(16 mi) to more than 60 km (37 mi). Regions that have high elevation generally have
thick crust. The crust beneath the Rocky Mountains of Colorado is commonly more than
45 km (28 mi) thick. The crust beneath low-elevation regions like the Gulf Coast of
Texas is thinner. If the crust is thinner than 30 to 35 km (18 to 20 mi), the area will
probably be below sea level, but it can still be part of the continent. Most islands are
volcanic mountains built on oceanic crust, but some are small pieces of continental crust.
Oceanic crust is thinner than continental crust and consists of denser rock than
continental crust. As a result, regions underlain only by oceanic crust are well below sea
level.
B. Major Features of Earth
Oceans cover around 70% of earth’s surface. Seven continents make up most of
the rest of the surface, and islands account for less than 2%. We are all familiar with the
continents and their remarkable diversity of landforms, from broad coastal plains to steep,
snow-capped mountains. Features of the ocean floor, not generally seen by people, are
just as diverse and include deep trenches and submarine mountain ranges. Islands exhibit
great diversity, too. Some are large and isolated, but other islands form in arc shapes,
ragged lines, or irregular clusters. What are the characteristics of each type of feature? In
this chapter, we will explore how each of these features forms, so it is worth some time
examining this map of the surface features of Earth.
This map shows large features on land and on the seafloor. The colors on land are
from images taken by satellites orbiting Earth and show vegetated areas (green), rocky
areas (brown), and sandy areas (tan). Greenland and Antarctica are white and light gray
because they are mostly covered with ice and snow. Ocean waters are removed in order
to reveal the slope of the seafloor. Colors of the ocean represent its depth, ranging from
light blue where the seafloor is shallow to darker blue where it is deep. Examine this map
and note the types of features you observe. Then, consider whether you have any ideas
about how each type of feature forms. Parts of the seafloor have mountains, the highest of
which form islands, such as Hawaii. Most mountains on the seafloor do not reach sea
level and are termed seamounts. Some islands, like Hawaii, and seamounts, are in long
belts, which we refer to as island and seamount chains. Other islands and seamounts are
isolated or form irregular clusters.
Some large islands, such as New Zealand, look like a small version of a continent.
Much of the ocean floor is moderately deep—3 to 5 km (9,800 to 16,000 ft)—and has a
fairly smooth surface. Such a smooth, deep part of the seafloor is an abyssal plain. A
mid-ocean ridge is a broad, symmetrical ridge that crosses an ocean basin. Most ridges
are 2 to 3 km (6,600–9,800 ft) higher than the average depth of the seafloor. One long
ridge, named the East Pacific Rise, crosses the eastern Pacific and heads toward North
America. Another occupies the middle of the Atlantic Ocean. Cracks and steps cross the
seafloor mostly at right angles to the mid-ocean ridges. Such a feature is an oceanic
fracture zone.
Some continents continue outward from the shoreline under shallow seawater
(light blue in this image) for hundreds of kilometers, forming submerged benches known
as continental shelves. Which coastlines have broad continental shelves, like those
surrounding Great Britain? All continents contain large interior regions with gentle
topography. Some continents have flat coastal plains, while others have mountains along
their edges. Some mountains, like the Ural Mountains, are in the middle of continents.
Most continental areas have elevations of less than 1 to 2 km (3,300 to 6,600 ft). Broad,
high regions, called plateaus, reach higher elevations, such as the Tibetan Plateau of
southern Asia. Continents also contain mountain chains and individual mountains. Mount
Everest, the highest point in the world, is almost 9 km (about 30,000 ft) in elevation.
Ocean trenches make up the deepest parts of the ocean. Some ocean trenches
follow the edges of continents, whereas others form isolated, curving troughs. Most
ocean trenches are in the Pacific Ocean. Why are they here? Crossing the seafloor are
curving chains of islands, each known as an island arc. Most of the islands in an island
arc are volcanoes, and many are active and dangerous. Most island arcs are flanked on
one side by an ocean trench. Offshore of the Mariana island arc, located south of Japan, is
the Mariana Trench, the deepest in the world at over 10.9 km (6.8 miles) deep—much
deeper below sea level than Mount Everest is above sea level. Some continents (such as
South America) are flanked by an ocean trench, but other continents, such as Australia
and Africa, have no nearby trenches.
The oceans contain several broad, elevated regions, each of which is an oceanic
plateau. The Kerguelen Plateau near Antarctica is one example, and another oceanic
plateau lies northeast of Australia. Mid-ocean ridges and their associated fracture zones
encircle much of the globe. In the Atlantic and Indian Oceans, they occupy a position
halfway between the adjacent continents.
C. Continents
Some continents have matching shapes that appear to fit together like the pieces
of a giant jigsaw puzzle. Alfred Wegener (1880 –1930), a German physical geographer
and meteorologist, observed the fit of these continents and tried to explain this and other
data with a hypothesis called continental drift. Wegener argued that the continents were
once joined together but later drifted apart. The hypothesis of continental drift was an
important historical step that led to current theories that explain the distribution and
shapes of the continents.
Fairly accurate world maps became available during the 1800s and scientists,
including Alfred Wegener, noted that some continents, especially the southern continents,
appeared to fit together. After considering many types of data, Wegener arrived at a
creative explanation for this pattern. This figure shows how the southern continents are
interpreted to have fit together 150 million years ago. In this figure, the continental
shelves are included because they are parts of continents, even though they are currently
underwater. In this arrangement, the bulge on the eastern side of South America fits
nicely into the embayment on the western coast of Africa.
The fit of the continents and other supporting evidence preserved in rocks and
fossils inspired Wegener and others to suggest that South America, Africa, Antarctica,
Australia, and most of India were once joined but later drifted apart. Even Madagascar
can fit into the puzzle. This “cut-and-paste” fit of the continents is intriguing and leads to
predictions for testing the hypothesis of continental drift. If continents were once joined,
they should have similar rocks and geologic structures. Geoscientists find such
similarities when they compare the rocks and structures in southern Australia with the
rocks and structures exposed around the edges of ice sheets on Antarctica. Similarly, the
geology of western Africa closely matches that of eastern South America, and these two
areas are adjacent to each other in Wegener’s reconstruction. Earth scientists gave the
name Gondwana to this theoretical combination of the southern continents into a single
large supercontinent.
Another piece of evidence supporting continental drift is the correspondence of
the fossils of plants and land animals on continents now several thousand kilometers
apart and separated by wide oceans. This figure illustrates that fossils of some land
animals exist on several continents that are now separated by wide oceans. The animals
lived more than 150 million years ago (abbreviated Ma) and are now extinct. These land
animals could not swim across the wide oceans that currently separate the continents.
Other key fossils linking the land areas of Gondwana are fossilized leaves of a
seedbearing plant that was widespread during late Paleozoic time (before 250 Ma). The
distribution of plant and animal fossils is consistent with the idea that the continents were
once joined. It was a key piece of evidence in favor of continental drift. The hypothesis of
continental drift provided an alternative to the hypothesis of land bridges, which
proposed that animals crossed the oceans on ridges that are now barely submerged.
Continental drift also explained why identical plant and animal fossils are found on
different continents. The plants and animals were originally on a single huge
supercontinent that later split into separate smaller continents. Two continents could
share plants and land animals before they split, but not after. Other fossil data suggest
that Antarctica was once farther north, away from the South Pole. Such data include coal
beds interpreted to have formed from plants that grew in warm-weather swamps. One
explanation is that Antarctica moved to its present polar location after the coal formed
more than 150 Ma.
Geoscientists studying continents in the Southern Hemisphere were puzzled by
evidence that ancient glaciers had once covered places that today are close to the equator,
and much too warm to have major glaciers. This rounded outcrop in South Africa has a
polished and scratched surface that is identical to those observed at the bases of modern
glaciers. This observation is surprising because South Africa is currently a fairly warm
and dry region without any glaciers. Sedimentary rocks above the polished surface
contain an unsorted collection of rocks of various sizes. Some of the rocks have scratch
marks, like those seen near modern-day glaciers. The scratch marks, or striations, on the
polished bedrock surface tell geoscientists the direction that glaciers moved across the
land as they gouged the bedrock. We interpret these scratch marks and other observations
as evidence that glaciers moved across the area about 280 Ma.
. The overall directions of glacial movement inferred from the scratch marks
made it seem as if the glaciers had come from the oceans, something that is not seen
today. Wegener discovered that these data made more sense when the continents were
pieced back together into a larger, ancient continent, as shown in this illustration.
According to this model, a polar ice cap was centered over South Africa and Antarctica
280 Ma, and the directions of glacial ice movement were those shown by the blue arrows.
D. Distribution of Earthquakes, Volcanoes, and Mountain Belts
Earthquakes and volcanoes are spectacular manifestations of our dynamic Earth.
Many of these are in distant places, but some are close to human settlements. The
distributions of earthquakes and volcanoes are not random, but instead define clear
patterns and show a close association with mountain belts and other regional features.
These patterns show important, broad-scale Earth processes.
On this map, yellow circles show the locations of moderate to strong earthquakes
that occurred over several recent decades. Observe the distribution of earthquakes before
reading on. What patterns do you notice? Which regions have many earthquakes and
which have few? Are earthquakes associated with certain types of features? Earthquakes
are not distributed uniformly across the planet. Most are concentrated in discrete belts,
such as one that runs along the western coasts of North and South America. Most
earthquakes in the oceans occur along the winding crests of mid-ocean ridges. Where the
ridges curve or zigzag, so do the patterns of earthquakes. Earthquakes are sparse in some
continental interiors but are abundant in others, like the Middle East, China, and near the
Himalaya.
Large areas of the seafloor, especially the abyssal plains, have few earthquakes.
Volcanically active islands, like Hawaii, in the middle of the Pacific Ocean, do have
earthquakes. Some continental edges experience many earthquakes, but other edges have
few. Earthquakes are common along the western coasts of South America and North
America, and these edges also have narrow continental shelves flanked by ocean
trenches. There are few earthquakes along the eastern coasts of the Americas, where the
continental shelves are wide. Ocean trenches and associated island arcs have numerous
earthquakes. In fact, many of the world’s largest and most deadly earthquakes occur near
ocean trenches. Recent examples were the large earthquakes that produced deadly ocean
waves in the Indian Ocean in 2004 and in Japan in 2011.
On the map below, orange triangles show the locations of volcanoes that have
been active in the last several million years. Observe the distribution of volcanoes and
note which areas have volcanoes and which have none. How does this distribution
compare with the distribution of earthquakes? Volcanoes, like earthquakes, are
widespread, but commonly occur in belts. One belt extends along the western coasts of
North and South America. Some volcanoes occur in the centers of oceans, such as the
volcanoes near Iceland. Iceland is a large volcanic island along the mid-ocean ridge in the
middle of the North Atlantic Ocean. Volcanoes occur along the western edge of the
Pacific Ocean, extending from north of Australia through the Philippines and Japan.
Many are part of island arcs, associated with ocean trenches and earthquakes.
Volcanic eruptions occur beneath the oceans, but this map shows only the largest
submarine volcanic mountains. Volcanism is widespread along mid-ocean ridges. Some
volcanoes form in the middle of continents, such as in the eastern part of Africa and in
China. This map shows the topography of Earth’s surface and seafloor, with high
elevations in brown, low land elevations in green, shallow seafloor in light blue, and deep
seafloor in dark blue. Using the three maps shown here, compare the distributions of
earthquakes, volcanoes, and high elevations. Identify areas where there are (1) mountains
but no earthquakes, (2) mountains but no volcanoes, and (3) earthquakes but no
volcanoes. Make a list of these areas, or mark the areas on a map.
Most Earthquakes and volcanoes occur in belts around Earth’s surface. Between
these belts are vast regions that have comparatively little of this activity. Where are these
belts of concentrated activity, and what explains this spatial distribution? What
underlying processes cause these observed patterns? These and other questions helped
lead to the theory of plate tectonics.
Examine the map below, which shows the locations of recent earthquakes (yellow
circles) and volcanoes (orange triangles). After noting the patterns, compare this map
with the lower map and then read the associated text. On the upper map, there are large
regions that have few earthquakes and volcanoes. These regions are relatively stable and
intact pieces of Earth’s outer layers. There are a dozen or so of these regions, each having
edges defined by belts of earthquakes and volcanoes. Earthquakes, volcanoes, and other
processes that deform the crust and mantle are called tectonic activity, or simply
tectonics. The belts of yellow and orange on the map are areas of active tectonics. The
regions between the belts are relatively stable.
This lower map shows how we currently interpret the patterns on the upper map.
Earth’s strong outer layer, the lithosphere, is broken into a dozen or so fairly rigid pieces,
called tectonic plates. This map shows names and boundaries of the larger plates. Spend
some time learning the names and locations of the larger plates. Compare the two maps
and note how the distribution of tectonic activity, especially earthquakes, outlines the
shapes of the plates. Earthquakes are a better guide to plate boundaries than are
volcanoes. Most, but not all, volcanoes are near plate boundaries, but many plate
boundaries have no volcanoes. Some earthquakes occur in the middle of plates, indicating
that the situation is more complicated than a simple plate-tectonic model, in part because
some parts of a plate are weaker than others. Forces can be transmitted through the strong
parts, causing weaker parts to break and slip, generating an earthquake within the plate.
Generally, most tectonic activity occurs near plate boundaries.
Plate boundaries have tectonic activity because plates are moving relative to one
another. For this reason, we talk about the relative motion of plates across a plate
boundary. Two plates can move away, toward, or sideways relative to one another,
resulting in three types of plate boundaries: divergent, convergent, and transform. At a
divergent boundary, two plates move apart relative to one another. In most cases, magma
fills the space between the plates. At a convergent boundary, two plates move toward one
another. A typical result is that one plate slides under the other. At a transform boundary,
two plates move horizontally past one another, as shown by the white arrows on the top
surface.
E. Divergent, Convergent and Transform Boundaries
At mid-ocean ridges, Earth’s tectonic plates diverge (move apart). Ridges are the
sites of many small to moderatesized earthquakes and much submarine volcanism, and
they record where two oceanic plates spread apart. On the continents, divergent motion
can split a continent into two pieces, forming a continental rift and perhaps a new ocean
basin as the pieces move apart.
Mid-ocean ridges are divergent plate boundaries where new oceanic lithosphere
forms as two oceanic plates move apart. These boundaries are also called spreading
centers because of the way the plates spread apart. A narrow trough, or rift, runs along
the axis of most mid-ocean ridges. The rift forms because large blocks of crust slip down
as spreading occurs. The movement causes fracturing, resulting in frequent small to
moderate-sized earthquakes. As the plates move apart, solid mantle in the asthenosphere
rises toward the surface. It partially melts as it rises, because decreasing pressures can no
longer confine the rock as a solid. The molten rock (magma) rises along narrow conduits,
accumulates in magma chambers beneath the rift, and eventually becomes part of the
oceanic lithosphere.
Much of the magma solidifies at depth, but some erupts onto the seafloor, forming
submarine lava flows. These eruptions create new ocean crust that is incorporated into the
oceanic plates as they move apart. Mid-ocean ridges are elevated above the surrounding
seafloor because they consist of hotter, less dense materials, including magma. Lower
density materials and thin lithosphere mean that the plate “floats” higher above the
underlying asthenosphere. The elevation of the seafloor decreases away from the ridge
because the rock cools and contracts, and because the less dense asthenosphere cools
enough to become part of the denser lithosphere.
Most divergent plate boundaries are beneath oceans, but a divergent boundary
may also form within a continent. This process creates a continental rift, such as the
Great Rift Valley in East Africa. Rifting can lead to seafloor spreading and formation of a
new ocean basin, following the progression shown here. The initial stage of continental
rifting commonly includes broad uplift of the land surface as mantle-derived magma
ascends into and pushes up the crust. The magma heats and can melt parts of the
continental crust, producing additional magma. Heating of the crust causes it to expand,
which results in further uplift. Stretching of the crust causes large crustal blocks to drop
down along faults, forming a continental rift, like in the Great Rift Valley. The
downdropped blocks may form basins that can trap sediment and water, resulting in
lakes.
Deep rifting causes solid mantle material in the asthenosphere to flow upward and
partially melt. The resulting magma may solidify beneath the surface or may erupt from
volcanoes and long fissures on the surface. The entire crust thins as it is pulled apart, so
the central rift becomes lower in elevation over time. The Rio Grande Rift, which runs
north-south through New Mexico, is another example of a continental rift in its early
stages. An ancient example is the Midcontinental Rift, a mostly buried feature that
underlies large parts of the north-central U.S.
If rifting continues, the continent splits into two pieces, and a narrow ocean basin
forms as seafloor spreading takes place. A modern example of this is the narrow Red Sea,
which runs between Africa and the Arabian Peninsula. As the edges of the continents
move away from the heat associated with active spreading, the thinned crust cools and
drops in elevation, eventually dropping below sea level. The continental margin ceases to
be a plate boundary. A continental edge that lacks tectonic activity is called a passive
margin.
With continuing seafloor spreading, the ocean basin becomes progressively wider,
eventually becoming a broad ocean like the modern-day Atlantic Ocean. The Atlantic
Ocean basin formed when North and South America rifted away from Europe and Africa,
following the sequence shown here. Continental edges on both sides of the Atlantic are
currently passive margins, lacking tectonic activity. Seafloor spreading continues today
along the ridge in the middle of the Atlantic Ocean, so the Americas continue to move
away from Europe and Africa.
East Africa and adjacent seas illustrate the different stages of continental rifting.
Here, a piece of continent has been rifted away from Africa, forming the Arabian
Peninsula, and another piece is in the early stages of possibly doing the same. Early
stages of rifting occur along the East African Rift, a long continental rift that begins near
the Red Sea and extends into central Africa. The rift is within an elevated (uplifted)
region and has several different segments, each featuring a downdropped rift. Some parts
of the rift contain large lakes.
The Red Sea represents the early stages of seafloor spreading. It began forming
about 50 million years ago when the Arabian Peninsula rifted away from Africa. The Red
Sea continues to spread and slowly grow wider. The East African Rift is topographically
lower than its flanks, but contains numerous active volcanoes, as represented by this
mountain within the rift. Volcanoes are especially common in the Afar region of
Ethiopia, located where the rift joins the Red Sea. The Afar region is one of the most
volcanically active regions on the planet.
Convergent boundaries form when two plates move toward each other.
Convergence can involve two oceanic plates, an oceanic plate and a continental plate, or
two continental plates. Oceanic trenches, island arcs, and Earth’s highest mountain belts
form at convergent boundaries. Many of Earth’s most dangerous volcanoes and largest
earthquakes also occur along these boundaries. Convergent boundaries are among the
most dangerous places on the planet!
Convergence of two oceanic plates forms an ocean-ocean convergent boundary.
One plate bends and slides beneath the other plate along an inclined zone. The process of
one plate sliding beneath another plate is subduction, and the zone around the downward-
moving plate is a subduction zone. Many large earthquakes occur in subduction zones.
An oceanic trench forms as the subducting plate moves down. Sediment and slices of
oceanic crust collect in the trench. This sheared, scraped-off material generally remains
completely submerged, but is exposed in a few islands, like Barbados, in the eastern
Caribbean. As the plate subducts, its temperature increases, releasing water from minerals
in the downgoing plate. This water causes melting in the overlying asthenosphere, and
the resulting magma is buoyant and rises into the overlying plate.
Some magma erupts, initially under the ocean and later as dangerous, explosive
volcanoes that rise above the sea. With continued activity, the erupted lava and exploded
volcanic fragments construct a curving belt of islands in an island arc. An example is the
arc-shaped belt of the Aleutian Islands of Alaska. The area between the island arc and the
ocean trench accumulates sediment, most of which comes from volcanic eruptions and
from the erosion of volcanic materials in the arc. Magma that solidifies at depth adds to
the volume of the crust. Over time, the crust gets thicker and becomes transitional in
character between oceanic and continental crust. Initially separate volcanic islands join to
form more continuous strips of land, as occurred to form the island of Java in Indonesia.
The convergence of an oceanic and a continental plate forms an ocean-continent
convergent boundary. Along this boundary, the denser oceanic plate subducts beneath the
more buoyant continental plate. An oceanic trench marks the plate boundary and receives
sediment from the adjacent continent. Again, sediment and other material are scraped off
the oceanic plate, forming a wedge of highly sheared material near the trench. Volcanoes
form on the surface of the overriding continental plate in the same way the volcanoes
form in an ocean-ocean convergent boundary. These volcanoes erupt, often violently,
producing large amounts of volcanic ash, lava, and mudflows, which pose a hazard for
people who live nearby. Examples include large volcanoes of the Andes of South
America and the Cascade Range of Washington, Oregon, northern California, and
southern British Columbia.
Compression associated with the convergent boundary squeezes the crust for
hundreds of kilometers into the continent. The crust deforms and thickens, resulting in
uplift of the region. Uplift and volcanism may produce a high mountain range, such as
the Andes. Magma forms by melting of the asthenosphere above the subduction zone,
due to fluids brought in by the subducted slab. The magma can solidify at depth, rise into
the overlying continental crust before solidifying, or reach the surface and cause a
volcanic eruption.
Volcanoes surround the Pacific Ocean, forming the Pacific Ring of Fire, as shown
in the map below. The volcanoes extend from the southwestern Pacific, through the
Philippines, Japan, and Alaska, and then down the western coasts of North and South
America. The Ring of Fire results from subduction on both sides of the Pacific Ocean. .
In the Pacific, new oceanic lithosphere forms along a mid-ocean ridge, the East Pacific
Rise. The Pacific plate is west (left) of this boundary and several smaller plates are to the
east (right). Once formed, new lithosphere moves away from the ridge as seafloor
spreading continues. Oceanic lithosphere of the Pacific plate and smaller plates subducts
beneath the Americas, forming oceanic trenches on the seafloor and volcanoes on the
overriding, mostly continental, plates. Subduction of oceanic lithosphere also occurs to
the west, beneath Japan and island arcs of the western Pacific. More oceanic plate is
subducted than is produced along the East Pacific Rise, so the width of the Pacific Ocean
is shrinking with time.
Two continental masses may converge along a continentcontinent convergent
boundary. The large plate in the figure to the right is partly oceanic and partly
continental. The oceanic part is being subducted to the right, under another continent at
an ocean-continent convergent boundary. As the oceanic part of the plate continues to
subduct, the two continents approach each other. Magmatic activity occurs in the
overriding plate above the subduction zone. The edge of the approaching continent has no
such activity because it is not at a plate boundary yet. When the converging continent
arrives at the subduction zone, it may partially slide under the other continent or simply
clog the subduction zone as the two continents collide. Because the two continents are
thick and have the same density, neither can be easily subducted beneath the other and
into the asthenosphere. Along the boundary, faults slice up the continental crust, stacking
one slice on top of another. Continental collisions form enormous mountain belts and
high plateaus, such as the Himalaya and Tibetan Plateau of southern Asia. The Himalaya
and Tibetan Plateau are still forming today, rising at a rate of about 5 mm/yr, as the
continental crust of India collides with the southern edge of the Eurasian plate.
At transform boundaries, plates slip horizontally past each other along transform
faults. In the oceans, transform faults are associated with mid-ocean ridges. Transform
faults combine with spreading centers to form a zigzag pattern on the seafloor. A
transform fault can link different types of plate boundaries, such as a mid-ocean ridge and
an ocean trench. Some transform boundaries occur beside or within a continent, sliding
one large crustal block past another, as occurs along the San Andreas fault in California.
To understand the zigzag character of mid-ocean ridges, examine how the two parts of
this pizza have pulled apart, just like two diverging plates. The break in the pizza did not
follow a straight line. It took jogs to the left and the right, following cuts where the pizza
was the weakest. Openings created where the pizza pulled apart represent the segments of
a mid-ocean ridge that are spreading apart. However, unlike a pizza, at a mid-ocean ridge,
no open gaps exist because new material derived from the underlying mantle fills the
space as fast as it opens, forming new oceanic crust. The openings are linked by breaks,
or faults, where the two parts of the pizza simply slide by one another. There are no gaps
along these breaks, only horizontal movement of one plate sliding past the other. Arrows
show the direction of relative motion. A fault that accommodates the horizontal
movement of one tectonic plate past another is a transform fault. The spreading direction
must be parallel to the transform faults and perpendicular to the spreading segments, so a
zigzag pattern is required to allow a plate boundary to be curved.
Mid-ocean ridges, such as this one in the South Atlantic Ocean, have a zigzag
pattern similar to the broken pizza. In this region, spreading occurs along north-south
ridges. The direction of spreading is east-west, perpendicular to the ridges. East-west
offsets are transform faults along which the two diverging plates simply slide past one
another, like the breaks in the pizza. These transform faults link the spreading segments
and have the relative motion shown by the white arrows. Transform faults along mid-
ocean ridges are generally perpendicular to the axis of the ridge. As in the pizza example,
transform faults are parallel to the direction in which the two plates are spreading apart.’
The zigzag pattern of mid-ocean ridges shows the alternation of spreading segments with
transform faults. In this example, the overall shape of the ridge mimics the edges of
Africa and South America and so was largely inherited from the shape of the original rift
that split the two continents apart.
Continuing outward from most transform faults is an oceanic fracture zone, which
is a step in the elevation of the seafloor. A fracture zone is a former transform fault that
now has no relative motion across it. It no longer separates two plates and instead is
within a single plate. Opposite sides of the fracture zone have different elevations
because they formed by seafloor spreading at differ ent times in the past, so they have
had different amounts of time to cool and subside after forming at the spread ing center.
Younger parts of the plate are warmer and higher than older parts, resulting in a step in
elevation on the seafloor (an oceanic fracture zone).
The Pacific seafloor and western North America contain several different
transform boundaries. The boundary between the Pacific plate and the North American
plate is mostly a transform boundary, with the Pacific plate moving northwest relative to
the main part of North America. The Queen Charlotte transform fault, shown as a long
green line, lies along the edge of the continent, from north of Vancouver Island to
southeastern Alaska. The zigzag boundary between the Pacific plate and the small Juan
de Fuca plate has three transform faults, shown here as green lines. These transform
faults link the ridge segments that are spreading (shown here as yellow lines).
The Mendocino fracture zone originated as a transform fault, but it is now entirely
within the Pacific plate and is no longer active. Oceanic crust to the north is higher
because it is younger than oceanic crust to the south. The zigzag boundary between the
Pacific plate and the small Juan de Fuca plate has three transform faults, shown here as
green lines. These transform faults link the ridge segments that are spreading (shown here
as yellow lines). The Mendocino fracture zone originated as a transform fault, but it is
now entirely within the Pacific plate and is no longer active. Oceanic crust to the north is
higher because it is younger than oceanic crust to the south. A transform fault links a
spreading center (between the Pacific plate and the Juan de Fuca plate) with the Cascadia
subduction zone and the San Andreas fault. The place where the three plate boundaries
meet is a triple junction. The Mendocino triple junction is the meeting place of two
different transform faults and a subduction zone. The San Andreas transform fault
extends from north of San Francisco to southeast of Los Angeles. The part of California
west of the fault is on the Pacific plate and is moving approximately 5 cm/yr to the
northwest relative to the rest of North America. South of this map area, the transform
boundary continues across southern California and into the Gulf of California.
Californians have a transform fault in their backyard, as well as many other
smaller faults. In central California, the San Andreas fault forms linear valleys, abrupt
mountain fronts, and lines of lakes. In the Carrizo Plain (⊲), the fault is a linear gash in
the topography. Some streams follow the fault and others jog to the right as they cross the
fault, recording relative movement of the two sides. In this view, the North American
plate is to the left, and the Pacific plate is to the right and is being displaced toward the
viewer at several centimeters per year.
F. Movement of Plates
The process of plate tectonics circulates material back and forth between the
asthenosphere and the lithosphere. Some asthenosphere becomes lithosphere at mid-
ocean spreading centers and then takes a slow trip across the ocean floor before going
back down into the asthenosphere at a subduction zone. Besides creating and destroying
lithosphere, this process is the major way that Earth transports heat to the surface.
How exactly do plates move? To move, an object must be subjected to a driving
force (a force that drives the motion). The driving force must exceed the resisting forces
— those forces that resist the movement, such as friction and any resistance from other
material that is in the way. What forces drive the plates? Subduc ting oceanic lithosphere
is more dense than astheno sphere, so gravity pulls the plate downward into the
asthenosphere. Slab pull is a significant force, and a plate being subducted generally
moves faster than a plate that is not being subducted. Subduction sets up other forces in
the mantle that can work with or against slab pull. The mid-ocean ridge is higher than the
seafloor away from the ridge because lithosphere near the ridge is thinner and hotter.
Gravity causes the plate to slide away from the topographically high ridge and push the
plate outward.
The astheno sphere, although a solid, is capable of flow. It experiences
convection, where hot material rises due to its lower density, while cold material sinks
because it is more dense. Hot material rises at mid-ocean ridges, cools, and eventually
sinks back into the asthenosphere at a subduction zone. Convection also occurs at centers
of upwelling mantle material called hot spots, and it can help or hinder the motion of a
plate.
Plates move at 1 to 15 cm/yr, about as fast as your fingernails grow. This map,
similar to one earlier in the chapter, shows velocities and relative motions along major
plate boundaries, based on long-term rates. Arrows indicate whether the plate boundary
has divergent (outward pointing), convergent (inward pointing), or transform (side by
side) motion. Modern technology allows direct measurement of plate motions using
satellites, lasers, and other tools. The measured directions and rates of plate motions are
consistent with our current concept of lithospheric plates and with the theory of plate
tectonics. Examine this map and determine which way your home is moving.
Global Positioning System (GPS) is an accurate location technique that uses small
radio receivers to record signals from several dozen Earth-orbiting satellites. By attaching
GPS receivers to sites on land and monitoring changes in position over time, Earth
scientists produce maps showing motions for each plate. Arrows point in the direction of
motion, and longer arrows indicate faster motion. Note the motions of different plates.
Africa is moving to the northeast, away from South America. North America is moving
westward and rotating counterclockwise in this view. The plate on which India rides is
converging with Asia to form the Himalaya. In all, these motions match predictions from
the theory of plate tectonics.
According to plate tectonics, oceanic crust forms from upwelling magma and
spreading at a mid-ocean ridge. The oceanic crust then moves away from the ridge with
further spreading. If this is so, the crust should be youngest near the ridge, where it was
just formed, and should be progressively older away from the ridge. Also, oceanic crust
near the ridge will not have had time to accumulate much sediment, but the sediment
cover should thicken outward from the ridge. Since 1968, ocean-drilling ships have
drilled hundreds of deep holes into the seafloor. Geoscientists use drill cores, represented
here by cylinders of rock and sediment, and other drilling results to measure the thickness
of sediment and examine the underlying volcanic rocks (basalt). Geoscientists analyze
samples of sediment, rock, and fossils to determine the age, character, and origin of the
materials. Drill core samples reveal that sediment is thin or absent on the ridge but
becomes thicker away from the ridge. Age determinations on fossils in the sediment and
from underlying volcanic rocks show that oceanic crust gets systematically older away
from mid-ocean ridges. Such drilling results strongly support the theory of plate
tectonics.
The map below shows the age of the seafloor, with letters marking the position of
some mid-ocean ridges (R) and trenches (T). Purple represents the oldest areas (about
180 million years), and the darkest orange represents very young oceanic crust. The
youngest oceanic crust is near mid-oceanridge spreading centers (R), as we expect from
plate tectonics. The oldest oceanic crust in any ocean is the most distant from mid-ocean
ridges. None is older than about 180 million years, because all older oceanic crust has
been subducted (destroyed). The oldest seafloor is near passive margins (P).
Paleomagnetism is the rock record of past changes in Earth’s magnetic field. The
magnetic field is strong enough to orient magnetism in certain minerals, especially the
magnetic, iron-rich mineral magnetite, in the direction of the prevailing magnetic field.
Magnetic directions preserved in volcanic rocks, intrusive rocks, and some sedimentary
rocks provide an important way to determine the rates of seafloor spreading.
Earth has a metallic iron core, which is composed of a solid inner core surrounded
by a liquid outer core. The liquid core flows and behaves like a dynamo (an electrical
generator), creating a magnetic field around Earth. The inner core transfers heat and less
dense material to the liquid outer core. This transfer causes liquid in the outer core to rise,
forming convection currents. These convection currents are limited to the outer core and
are not the same as those in the upper mantle. Movement of the molten iron is affected by
forces associated with Earth’s rotation. The resulting movement of liquid iron and
electrical currents generates Earth’s magnetic field.
The north and south magnetic poles have switched many times, typically
remaining either normal or reversed anywhere from 100,000 years to a few million years.
Scientists have constructed a magnetic timescale by isotopically dating sequences of
rocks that contain magnetic reversals. This geomagnetic polarity timescale then serves as
a reference to compare against other sequences of rocks. The timescale shows periods of
normal magnetization (N) in black and those of reversed magnetization (R) in white.
Variability in the spacing and duration of magnetic reversals produced a unique pattern
through time. Geoscientists can measure the pattern of reversals in a rock sequence and
compare this pattern to the magnetic timescale to see where the patterns match. This
allows an estimate of the age of the magnetic rock or sediment. Other age constraints,
including isotopic ages or fossils, are used to further refine the age of the magnetized
rocks. The magnetic timescale is best documented for the last 180 million years because
seafloor of this age is widely preserved, allowing measurements in many places.
In the 1950s, scientists discovered that the ocean floor displayed magnetic
variations in the form of matching magnetic stripes on either side of the mid-ocean ridge.
They interpreted the patterns to represent a magnetic field that had reversed its polarity,
an idea that led to the theory of plate tectonics. Magnetic patterns allow us to estimate the
ages of large areas of seafloor and to calculate the rates at which two diverging oceanic
plates spread apart. As the oceanic plates spread apart at a mid-ocean ridge, basaltic lava
erupts onto the surface or solidifies at depth. As the rocks cool, the orientation of Earth’s
magnetic field is recorded by the iron-rich mineral magnetite. In this example, the
magnetite records normal polarity (shown with a reddish color) at the time the rock
forms. Rocks forming all along the axis of the mid-ocean ridge will have the same
magnetic direction, forming a stripe of similarly magnetized rocks parallel to the ridge.
If the magnetic field reverses, any new rocks that form will acquire a reversed
polarity (shown in white). Once the rocks have cooled, they retain their original magnetic
direction, preserving the magnetic polarity in the seafloor. The new reversely magnetized
rocks form a stripe along the mid-ocean ridge, and the previously formed, normally
magnetized rocks have been split into stripes and have moved away from the ridge in
opposite directions. The magnetic poles have switched many times, and continued
seafloor spreading produces a pattern of alternating magnetic stripes on the ocean floor.
This pattern is strong enough to be detected by magnetic instruments towed behind a ship
or a plane.
As magnetic instruments are towed behind a ship, the strength of the magnetic
field is measured and plotted. Stronger measurements, representing rocks with normal
magnetization, plot high on the graph and are called positive magnetic anomalies. The
reverse magnetization of the rocks slightly weakens the measured magnetic signal and
will plot low on the graph, forming a negative magnetic anomaly. The seafloor patterns
are compared with the patterns on the geomagnetic polarity timescale to assign ages to
each reversal. We can simplify and visualize these data as reversely and normally
magnetized stripes on the seafloor, as shown in this cross section.
The magnetic patterns on the seafloor, in addition to magnetic measurements on
sequences of rocks and sediment on the seafloor and on land, demonstrate that Earth’s
magnetic field has reversed many times. Scientists are currently debating the possible
causes of the magnetic reversals, with most explanations attributing reversals to chaotic
flow in the molten outer core, which add to or subtract from the patterns caused by the
dynamo, disrupting the prevailing magnetic field and causing a reversal. What do you
think we might experience during a magnetic reversal in our lifetime?
G. Oceanic and Continental Hot Spots
Submarine Mountains, called seamounts, rise above the seafloor. In some places,
they reach the surface and make islands. In many cases, islands and seamounts form
linear chains, as in Hawaii. The seafloor also has relatively high and broad areas that are
oceanic plateaus. How are seamounts and oceanic plateaus formed? Linear chains of
islands and seamounts, and most clusters of islands in the oceans, have two key things in
common: they were formed by volcanism and they are near sites interpreted to be above
unusually high-temperature regions in the deep crust and upper mantle — hot spots. Hot
spots are interpreted to represent the surface manifestation of hot masses of material
rising from the deep mantle.
This figure shows how linear island and seamount chains can be related to a plate
moving over a hot spot. At a hot spot, hot mantle rises and melts, forming magma that
ascends into the overlying plate. The magma generated by a hot spot may solidify at
depth or form a volcanic mountain on the ocean floor. If the submarine volcano grows
high enough above the seafloor, it becomes a volcanic island. Each of the Hawaiian
Islands consists of volcanoes. Geomorphologists consider the hot spot to be currently
below or near the eastern side of the Big Island, near Kilauea volcano. If the plate above
the hot spot is moving relative to the hot spot, the volcanic island can move off the hot
spot. As it does, that part of the plate cools and subsides, so volcanoes that start out as
islands may sink beneath the sea to become seamounts. As an island sinks, erosion can
bevel off its top, forming a flat-topped seamount.
As a plate moves over a hot spot, volcanism constructs a chain of volcanic islands
and seamounts, each created when it was over the hot spot. According to this model,
volcanoes above the hot spot may be erupting today, those close to but not above the hot
spot are relatively young, and those farthest from the hot spot are older. If a plate is not
moving or is moving very slowly, the hot spot forms a cluster of volcanic islands and
seamounts instead of a linear chain. The Galápagos, a cluster of volcanic islands in the
eastern Pacific, is interpreted to be above such a hot spot.
Some large regions of the seafloor rise a kilometer or more above their
surroundings, forming oceanic plateaus. These plateaus are largely composed of flood
basalts, like the seafloor in general. How do these form? This perspective shows the
Kerguelen oceanic plateau, which rises above the surrounding seafloor in the southern
Indian Ocean. The plateau is several thousand kilometers long, but it only reaches sea
level in a few small islands. The small sliver of land showing in the lower right corner is
part of Antarctica. Geoscientists interpret oceanic plateaus as forming at hot spots, above
rising hot mantle, called mantle plumes. The plumes travel through the mantle as solid
masses, not liquids. When the top of a plume encounters the base of the lithosphere, it
causes widespread melting. Submarine basalts pour out onto the seafloor through fissures
and central vents. Immense volumes of basalt (as much as 50 million cubic kilometers)
erupt onto the seafloor over millions of years. This volcanism creates a broad, high
oceanic plateau.
Hot spots have created many Pacific islands that we associate with tropical
paradises and exotic destinations, like Tahiti and the Galápagos. Hawaii is the most
famous island chain formed by movement of a plate over a hot spot, but several other
linear island and seamount chains, in both the Atlantic and Pacific, formed in the same
manner. We show oceanic and continental hot spots on the map below, but we discuss
continental hot spots on the following two pages. On this map, red dots show the
locations of likely hot spots, many of which are located at the volcanically active ends of
linear island chains. There is great debate, however, about which areas really are hot
spots and how hot spots form. The dark gray areas in the oceans represent linear island
chains, clumps of islands, and oceanic plateaus, similar to this high area around Iceland,
which is over a hot spot.
The Ontong Java Plateau is the largest oceanic plateau on Earth, covering millions
of square kilometers, nearly 1% of Earth’s surface area. It formed in the middle of the
Pacific Ocean 120 Ma and is no longer near the hot spot that produced it. Volcanic
islands near Tahiti define northwesttrending chains that are forming over several hot
spots. In each chain, the islands to the northwest are older than those to the southeast,
indicating that the Pacific plate is moving to the northwest relative to the underlying
source of magma. The Galápagos is a clump of volcanic islands west of South America.
The western islands, shown in the satellite image to the lower left, are volcanically active
and have erupted within the past several years. Eruptions build larger volcanoes and
smaller volcanic cones, as shown in the photograph below. Tristan da Cunha, a volcanic
island in the South Atlantic Ocean, marks a hot spot just east of the Mid-Atlantic Ridge.
Volcanism associated with the hot spot created a large submarine ridge (shown in gray)
that tracks the motion of the African plate over the hot spot. The Kerguelen Plateau, in
the southern Indian Ocean, is the second largest oceanic plateau in the world. It mostly
consists of basalt and was formed in several stages during the late Mesozoic (between
115 and 85 Ma).
A hot spot within continental plate is marked by high elevations, abundant
volcanism, and continental rifting. Hot spots can facilitate complete rifting and separation
of a continent into two pieces and can help determine where the split occurs. Several
continental hot spots are active today. Hot spots are volcanic areas interpreted to be
above rising mantle plumes. Continental hot spots are associated with certain
characteristics, including high elevations, volcanism, and the presence of rifts. Two
examples are the Afar region of East Africa and the Yellowstone region of the western
U.S.
Continental hot spots have high elevations largely because of heating and thinning
of the lithosphere by a rising plume of hot mantle. Many geoscientists interpret the Afar
region of eastern Africa to be located above a hot spot that is currently active. The East
African Rift is within the African plate. It may or may not evolve into a full rift that
fragments the continent into two parts and that leads to seafloor spreading. Near the hot
spot, the Arabian Peninsula has pulled away from Africa along the Red Sea and the Gulf
of Aden. Beneath these seas, seafloor spreading generates new oceanic crust. The
divergent plate boundary is along the rift down the middle of the Red Sea and then turns
eastward, following the zigzag oceanic ridge through the Gulf of Aden. The Red Sea,
Gulf of Aden, and East African Rift meet in the Afar region of Ethiopia, branching off
like three spokes on a wheel. The Afar region is among the most volcanically active areas
on Earth and has experienced recent volcanic eruptions. Volcanism has been so prolific
here that it has created a triangular area of new land in the corner of Africa from which
the Arabian Peninsula pulled away.
Yellowstone is located in Wyoming, Idaho, and Montana and sits in a region that
is higher in elevation than surrounding areas. The Snake River Plain of southern Idaho is
underlain by thick sequences of basalt and other volcanic rocks. It is the site of recent
eruptions at Craters of the Moon National Monument. Three large volcanic centers, each
issuing huge eruptions of volcanic ash, erupted in Yellowstone in the last 2.1 million
years. Eruptions are not occurring today, but heat from hot volcanic rock and underlying
magma drives the hot springs, geysers, and thermal pools for which Yellowstone is
famous. It could erupt again! Yellowstone is interpreted to mark the present location of
the hot spot, whereas the Snake River Plain records the track of North America as it
moved southwest over the hot spot. Forming at the same time was the Basin and Range
Province of Utah and Nevada, a broad continental rift adjacent to Yellowstone. The
alternating valleys and mountains resulted from faults that uplifted some blocks of crust
and downdropped others.
Many continental hot spots underwent a similar sequence of events. They started
with doming and ended with the formation of a new continental margin and a new ocean
formed by seafloor spreading. Hot spots mark where a mostly solid, hot mass rises,
probably from the lower mantle, and encounters the base of the lithosphere. The rising
material melts as a result of less pressure and also causes melting of nearby lithosphere.
As the upper mantle and crust heat up, a broad, domal uplift forms on the surface.
Doming is accompanied by stretching of the crust, which commonly begins to break apart
along three rifts that radiate out from the hot spot. Some mantle-derived magma escapes
to the surface and erupts as voluminous, very fluid basalts called flood basalts. Granitic-
composition magmas form where mantlederived magma causes melting of the continental
crust. All three parts, or arms, of the rift are bordered by faults, which downdrop long
fault blocks. The downdropped blocks form basins that contain lakes and are partially
filled by sediment and rift-related volcanic rocks.
Complete rifting of the continent occurs along two arms of the rift. This results in
a new continental margin and seafloor spreading in the new ocean basin. At the onset of
spreading, the edge of the continent is uplifted because the lithosphere is heated and
thinned due to the rifting. The third arm of the rift begins to become less active and fails
to break up the continent into more pieces. This failed rift is lower than the surrounding
continent and commonly becomes the site of major rivers. As seafloor spreading
continues, the generation of new oceanic lithosphere causes the mid-ocean ridge to move
farther out to sea. The continental margin cools and subsides and is covered by marine
sediment on the newly formed continental shelf. This continental margin is no longer a
plate boundary and is now a passive margin. Sediment transported by streams down the
failed rift will form a delta at the bend in the continent. This is currently occurring along
the western coast of Equatorial Africa at the large inward bend in the coast (see the figure
and text below).
H. Continents
Continents are among the largest features of our planet, but what are they, how
did they form, and does a continent remain the same size over long time periods?
Continents have interesting histories, which at times involved splitting apart or joining
together. Through geologic time, continents overall have tended to grow larger as they
collide with and incorporate island arcs, oceanic plateaus, seamounts, and other tectonic
features. When people think of a continent, most envision its shape as seen by its present-
day coastline. This outline, however, changes as sea level goes up or down, as it has
many times in the distant past. We should therefore consider a slightly different definition
of a continent — one that indicates a less transient outline.
Notice, however, that surrounding the land is an area of relatively shallow ocean,
represented on this figure by light blue. This blue shelf surrounding the continent is
similar to the rest of the continent, except that it is barely under water. Earth scientists
consider this shallow, continental shelf to be part of the Australian continent, putting the
edge of the continent at the outer edge of the continental shelf, not at the coastline We
can represent continents, and the rest of Earth’s land and seafloor, in a graph that plots
elevation versus the percentage of land or seafloor that is above that elevation (or depth).
This graph, called a hypsometric curve, shows that somewhat less than 30% of Earth is
continent at this time, with most of the rest being deep ocean. The major change between
the continents and deep oceans occurs not at the shoreline, but across continental slopes.
This graph illustrates that the continental shelves are more akin to the continents than to
the deep seafloor.
Examine this figure showing various tectonic features within and next to a central
plate being consumed by subduction from both sides. If we predict where each feature
will go as the plate motions continue, we expect that some of these features will collide
with each other and with the continent to the right. When this occurs, they may be added
to the front edge of the continent. A piece of exotic crust added to a continent is called a
tectonic terrane (note the spelling). Many terranes consist of volcanic and volcanic-
related rocks that formed as island arcs. Island arcs often become terranes because they
move across the ocean, potentially traveling long distances until they collide with, and
become part of, another landmass, like a continent. Some terranes have more continental
characteristics, specifically thick granitic crust with continental sedimentary rocks. Such
terranes generally represent pieces that were sliced or rifted off another continent and
then tectonically transported until they collided with the edge of a continent.
Some terranes represent oceanic islands, oceanic plateaus, and other types of
oceanic crust. Islands and plateaus have thicker-than-normal oceanic crust, so they are
less likely to be subducted and more likely to be added to the edge of a continent. The
oceanic plateau shown here is headed for the trench to the right and a collision with the
edge of the adjacent continent. For a terrane to become attached to a continent, it
typically enters a subduction zone, where it is scraped off the sub ducting plate and
tectonically added to the continent. Through this process of adding terranes along
subduction zones, a continent grows. In fact, incorporating terranes along their edges is
probably the main way most continents grow, and this process was very important in
constructing the continents we have today.
Many continents display a similar pattern, with a central region of older igneous
and metamorphic rocks surrounded by a relatively thin veneer of younger, nearly flat-
lying sedimentary layers. Many continents, including North America, have a central
region called a continental shield. A shield consists of relatively old metamorphic and
igneous rocks, often simply called crystalline rocks or the crystalline basement. The rocks
exposed in the shield represent the kinds of rocks that underlie much of the continent. In
North America, the continental shield is mostly in Canada and is called the Canadian
Shield. Surrounding the shield is a broad region called the continental platform. It is
characterized by nearly horizontal sedimentary rocks that were deposited on top of the
older, lower rock. The sedimentary layers commonly have been gently warped and then
eroded, which results in higher and lower rocks being exposed at the surface from place
to place. In the U.S., the entire center of the country, between the Rocky and Appalachian
Mountains, is a continental platform.
The boundary between the flat-lying platform sedimentary rocks and the
underlying crystalline rocks is an old, buried erosion surface (an unconformity), which
separates rocks with very different ages and histories. Sedimentary rocks in the interior
generally have few major geologic structures, but some regions, such as the continental
shield, have been gently warped upward, and others are warped downward, forming a
low area called a basin. A large, ancient basin like this underlies nearly all of Michigan.
Most continents have at least one edge where elevation of the land slowly
decreases toward the coast until it flattens out in a low plain barely above sea level. This
low region is a coastal plain and is underlain by sedimentary units, many of which were
deposited quite recently in geologic terms. In the U.S., the coastal plain warps around the
southern and eastern edge of North America, from Texas to Florida to New Jersey.
I. Landscapes
Earth’s landscapes reveal the interplay between internal processes that those
originating from within the Earth — and external processes — those imposed on the
Earth by its envelope of moving fluids (atmosphere and hydrosphere). Internal processes
tend to be constructive, such as when they construct a volcano or cause uplift of a
mountain range. External processes, especially erosion, tend to wear away parts of the
landscape, but the eroded materials are deposited in other areas. Internal and external
processes interact in some surprising ways.
Internal processes arise from within the Earth and are manifested in phenomena
such as volcanism, earthquakes, deformation, and mountain building. They are largely
driven by the planet’s internal heat. External processes, like weathering and erosion,
occur on or near the surface and are mostly driven by gravity and the energy from the
Sun and the resulting movement of air and moisture. Most internal processes are
expressed as tectonics. Volcanism and mountain building can build new landscape
features, such as a volcano or mountain, or can modify existing landscapes by faulting
and uplift. Once rocks and other materials are exposed at the surface, weathering (an
external process) begins to loosen pieces and round off corners and in general acts to
disintegrate the rocks. It creates clasts that can be moved. Clasts, clay, and other materials
produced by weathering are then eroded away and transported by streams, waves, ice,
wind, and gravity. Erosion, transport, and eventual deposition are all external processes.
External processes can disintegrate and transport materials in a landscape, but
tectonics can influence the rates at which different external processes are operating. The
current arrangement and topography of the continents and ocean basins help guide wind
and ocean currents that redistribute heat and moisture around Earth. Tectonics can change
topography, which changes the local or regional climate, the rate of weathering, and other
factors. Mountain ranges intercept wind and water vapor, causing orographic effects that
concentrate rainfall and cause rain shadows and other climatic effects. Elevation and
moisture in turn affect rates of weathering and erosion Tectonics can slowly displace
water from seas, flooding low parts of continents. Such slow regional flooding, or more
rapid local earthquake-related tectonic subsidence, can change local climates. Volcanic
activity releases CO2 and water vapor, which cause atmospheric warming. Volcanic ash
and SO2 gas from volcanoes reflect solar radiation, which may cause regional or global
cooling.
As a tectonic plate moves, it changes position relative to Earth’s climatic zones.
Plate movements open and close channels that connect oceans, thereby changing ocean
currents. Tectonics can uplift mountain ranges, form new ocean basins, or bring
continents together. These changes can alter a region’s elevation, temperature, wind flow
patterns, and the amount and frequency of precipitation, all of which influence the rate of
external processes. To illustrate how tectonics can influence external processes, we use
the three maps below to show the geography of North America at different times: 260
Ma, 75 Ma, and 20 Ma, from left to right. Examine each map and think about how the
configuration of land and water might change regional climates. Also, consider how the
mountains would influence wind patterns and the amount of precipitation. In the first
map, North America was just north of the equator so would experience northeast trade
winds. In the other two maps, it was farther north and so would be under the influence of
westerlies, as it is today. Only the general patterns are important here.
At about 260 Ma, North America was part of the supercontinent of Pangaea. The
newly formed Appalachian Mountains were probably a huge, high mountain range, much
like today’s Himalaya. They blocked easterly trade winds, causing a rain shadow with
deserts over the center of the continent, the same areas (e.g., upper Midwest) that receive
abundant rainfall today. At 75 Ma, shallow inland seas inundated the continent, bringing
a warm, wet climate far inland, covering most of the continent. Tectonics along the west
coast formed a large and fairly continuous belt of mountains inland from the coast. These
changed the wind patterns and regional elevations, which affected the amounts, rates, and
seasonality of rainfall, weathering, and erosion. In the last 20 Ma, mountains along the
West Coast, such as the Sierra Nevada and Cascades, increasingly blocked prevailing
westerly winds, causing increased precipitation along the coast and the western slopes of
the mountains, but a rain shadow with deserts inland. The patterns of humidity, rainfall,
weathering, and erosion probably started to resemble those of the present day.
In actively uplifting mountain belts, hillslopes indicate the interplay between
internal processes, which are uplifting the mountains and causing slopes to be steeper,
and external processes that are eroding the rising mountains and causing slopes to be less
steep. An example from the south-facing slopes of the Himalaya nicely illustrates this
interplay. As mountains are uplifted, rocks at the surface are removed by water, ice,
landslides, and the slower movement of material downhill. In some mountain ranges,
erosion is almost as rapid as uplift and so limits the heights the mountains can achieve.
Also, uplift of the mountains leads to regional climate changes, which may act to lower
the range through increased erosion. In other areas, uplift seems to outpace erosion, so
the mountains get higher and generally steeper his graph is a topographic profile,
showing the slope of the land. Mountain slopes of central Nepal have a curved
topographic profile (⊳ ) that may indicate a near balance between uplift and erosion.
